'\" te
.\"  Copyright 1989 AT&T Copyright (c) 2005, Sun Microsystems, Inc. All Rights Reserved
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.TH DLOPEN 3C "May 16, 2020"
.SH NAME
dlopen, dlmopen \- gain access to an executable object file
.SH SYNOPSIS
.nf
#include <dlfcn.h>
#include <link.h>

\fBvoid *\fR\fBdlopen\fR(\fBconst char *\fR\fIpathname\fR, \fBint\fR \fImode\fR);
.fi

.LP
.nf
\fBvoid *\fR\fBdlmopen\fR(\fBLmid_t\fR \fIlmid\fR, \fBconst char *\fR\fIpathname\fR, \fBint\fR \fImode\fR);
.fi

.SH DESCRIPTION
The \fBdlopen()\fR function makes an executable object file available to a
running process. \fBdlopen()\fR returns to the process a \fIhandle\fR that the
process can use on subsequent calls to \fBdlsym\fR(3C), \fBdladdr\fR(3C),
\fBdlinfo\fR(3C), and \fBdlclose\fR(3C). The value of this \fIhandle\fR should
not be interpreted in any way by the process. The \fIpathname\fR argument is
the path name of the object to be opened. A path name containing an embedded
\fB\&'/'\fR is interpreted as an absolute path or relative to the current
directory. Otherwise, the set of search paths currently in effect by the
runtime linker are used to locate the specified file. See \fBNOTES\fR.
.sp
.LP
The \fBdlopen()\fR function also loads any dependencies recorded within
\fIpathname\fR. These dependencies are searched in the order in which the
dependencies were loaded to locate any additional dependencies. This process
continues until all the dependencies of \fIpathname\fR are loaded. This
dependency tree is referred to as a group.
.sp
.LP
If the value of \fIpathname\fR is \fB0\fR, \fBdlopen()\fR provides a
\fIhandle\fR on a set of global symbol objects. These objects consist of the
original program image file, any dependencies loaded at program startup, and
any objects loaded using \fBdlopen()\fR with the \fBRTLD_GLOBAL\fR flag.
Because the latter set of objects can change during process execution, the set
identified by \fIhandle\fR can also change dynamically.
.sp
.LP
The \fImode\fR argument describes how \fBdlopen()\fR operates on \fIpathname\fR
with respect to the processing of reference relocations. The \fImode\fR also
affects the scope of visibility of the symbols provided by \fIpathname\fR and
its dependencies. This visibility can affect how the resulting \fIhandle\fR is
used.
.sp
.LP
When an object is loaded, the object can contain references to symbols whose
addresses are not known until the object is loaded. These references must be
relocated before the symbols can be accessed. References are categorized as
either \fBimmediate\fR or \fBlazy\fR. Immediate references are typically
references to data items used by the object code. Immediate references include
pointers to functions and calls to functions made from position-dependent
shared objects. Lazy references are typically calls to global functions that
are made from position-independent shared objects. The \fImode\fR argument
governs when these references take place. The \fImode\fR argument can be one of
the following values:
.sp
.ne 2
.na
\fB\fBRTLD_LAZY\fR\fR
.ad
.RS 13n
Only immediate symbol references are relocated when the object is first loaded.
Lazy references are not relocated until a given function is called for the
first time. This value for \fImode\fR should improve performance, since a
process might not require all lazy references in any given object. This
behavior mimics the normal loading of dependencies during process
initialization. See \fBNOTES\fR.
.RE

.sp
.ne 2
.na
\fB\fBRTLD_NOW\fR\fR
.ad
.RS 13n
All necessary relocations are performed when the object is first loaded. This
process might waste some processing if relocations are performed for lazy
references that are never used. However, this mode ensures that when an object
is loaded, all symbols referenced during execution are available. This behavior
mimics the loading of dependencies when the environment variable
\fBLD_BIND_NOW\fR is in effect.
.RE

.sp
.LP
See the \fILinker and Libraries Guide\fR for more information about symbol
references.
.sp
.LP
The visibility of symbols that are available for relocation can be affected by
\fImode\fR. To specify the scope of visibility for symbols that are loaded with
a \fBdlopen()\fR call, \fImode\fR should be a bitwise-inclusive OR with one of
the following values:
.sp
.ne 2
.na
\fB\fBRTLD_GLOBAL\fR\fR
.ad
.RS 15n
The object's global symbols are made available for the relocation processing of
any other object. In addition, symbol lookup using \fBdlopen(0,\fR
\fImode\fR\fB)\fR and an associated \fBdlsym()\fR allows objects that are
loaded with \fBRTLD_GLOBAL\fR to be searched.
.RE

.sp
.ne 2
.na
\fB\fBRTLD_LOCAL\fR\fR
.ad
.RS 15n
The object's globals symbols are only available for the relocation processing
of other objects that include the same group.
.RE

.sp
.LP
The program image file and any objects loaded at program startup have the mode
\fBRTLD_GLOBAL\fR. The mode \fBRTLD_LOCAL\fR is the default mode for any
objects that are acquired with \fBdlopen()\fR. A local object can be a
dependency of more than one group. Any object of mode \fBRTLD_LOCAL\fR that is
referenced as a dependency of an object of mode \fBRTLD_GLOBAL\fR is promoted
to \fBRTLD_GLOBAL\fR. In other words, the \fBRTLD_LOCAL\fR mode is ignored.
.sp
.LP
Any object loaded by \fBdlopen()\fR that requires relocations against global
symbols can reference the symbols in any \fBRTLD_GLOBAL\fR object. Objects of
this mode are at least the program image file and any objects loaded at program
startup. A loaded object can also reference symbols from itself, and from any
dependencies the object references. However, the \fImode\fR parameter can also
be a bitwise-inclusive \fBOR\fR with one of the following values to affect the
scope of symbol availability:
.sp
.ne 2
.na
\fB\fBRTLD_GROUP\fR\fR
.ad
.RS 15n
Only symbols from the associated group are made available for relocation. A
group is established from the defined object and all the dependencies of that
object. A group must be completely self-contained. All dependency relationships
between the members of the group must be sufficient to satisfy the relocation
requirements of each object that defines the group.
.RE

.sp
.ne 2
.na
\fB\fBRTLD_PARENT\fR\fR
.ad
.RS 15n
The symbols of the object initiating the \fBdlopen()\fR call are made available
to the objects obtained by \fBdlopen()\fR. This option is useful when
hierarchical \fBdlopen()\fR families are created. Although the parent object
can supply symbols for the relocation of this object, the parent object is not
available to \fBdlsym()\fR through the returned \fIhandle\fR.
.RE

.sp
.ne 2
.na
\fB\fBRTLD_WORLD\fR\fR
.ad
.RS 15n
Only symbols from \fBRTLD_GLOBAL\fR objects are made available for relocation.
.RE

.sp
.LP
The default modes for \fBdlopen()\fR are both \fBRTLD_WORLD\fR and
\fBRTLD_GROUP\fR. If an object is requires additional modes, the \fImode\fR
parameter can be the bitwise-inclusive OR of the required modes together with
the default modes.
.sp
.LP
The following modes provide additional capabilities outside of relocation
processing:
.sp
.ne 2
.na
\fB\fBRTLD_NODELETE\fR\fR
.ad
.RS 17n
The specified object is tagged to prevent its deletion from the address space
as part of a \fBdlclose()\fR.
.RE

.sp
.ne 2
.na
\fB\fBRTLD_NOLOAD\fR\fR
.ad
.RS 17n
The specified object is not loaded as part of the \fBdlopen()\fR. However, a
valid \fIhandle\fR is returned if the object already exists as part of the
process address space. Additional modes can be specified as a bitwise-inclusive
OR with the present mode of the object and its dependencies. The
\fBRTLD_NOLOAD\fR mode provides a means of querying the presence or promoting
the modes of an existing dependency.
.RE

.sp
.LP
The default use of a \fIhandle\fR with \fBdlsym()\fR allows a symbol search to
inspect all objects that are associated with the group of objects that are
loaded from \fBdlopen()\fR. The \fImode\fR parameter can also be a
bitwise-inclusive OR with the following value to restrict this symbol search:
.sp
.ne 2
.na
\fB\fBRTLD_FIRST\fR\fR
.ad
.RS 14n
Use of this \fIhandle\fR with \fBdlsym()\fR, restricts the symbol search to the
first object associated with the \fIhandle\fR.
.RE

.sp
.LP
An object can be accessed from a process both with and without
\fBRTLD_FIRST\fR. Although the object will only be loaded once, two different
\fIhandle\fRs are created to provide for the different \fBdlsym()\fR
requirements.
.sp
.LP
The \fBdlmopen()\fR function is identical to \fBdlopen()\fR, except that an
identifying link-map ID \fI(lmid)\fR is provided. This link-map ID informs the
dynamic linking facilities upon which link-map list to load the object. See the
\fILinker and Libraries Guide\fR for details about link-maps.
.sp
.LP
The \fIlmid\fR passed to \fBdlmopen()\fR identifies the link-map list on which
the object is loaded. This parameter can be any valid \fBLmid_t\fR returned by
\fBdlinfo()\fR or one of the following special values:
.sp
.ne 2
.na
\fB\fBLM_ID_BASE\fR\fR
.ad
.RS 15n
Load the object on the applications link-map list.
.RE

.sp
.ne 2
.na
\fB\fBLM_ID_LDSO\fR\fR
.ad
.RS 15n
Load the object on the dynamic linkers (\fBld.so.1\fR) link-map list.
.RE

.sp
.ne 2
.na
\fB\fBLM_ID_NEWLM\fR\fR
.ad
.RS 15n
Cause the object to create a new link-map list as part of loading. Objects that
are opened on a new link-map list must express all of their dependencies.
.RE

.SH RETURN VALUES
The \fBdlopen()\fR function returns \fINULL\fR if \fIpathname\fR cannot be
found, cannot be opened for reading, or is not a shared object or a relocatable
object. \fBdlopen()\fR also returns \fINULL\fR if an error occurs during the
process of loading \fIpathname\fR or relocating its symbolic references. See
\fBNOTES\fR. Additional diagnostic information is available through
\fBdlerror()\fR.
.SH USAGE
The \fBdlopen()\fR and \fBdlmopen()\fR functions are members of a family of
functions that give the user direct access to the dynamic linking facilities.
This family of functions is available only to dynamically-linked processes. See
the \fILinker and Libraries Guide\fR.
.SH ATTRIBUTES
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT\(miLevel	MT\(miSafe
.TE

.SH SEE ALSO
.BR ld (1),
.BR ld.so.1 (1),
.BR dladdr (3C),
.BR dlclose (3C),
.BR dldump (3C),
.BR dlerror (3C),
.BR dlinfo (3C),
.BR dlsym (3C),
.BR attributes (7),
.BR standards (7)
.sp
.LP
\fILinker and Libraries Guide\fR
.SH NOTES
If \fIpathname\fR has dependencies on other objects, these objects are
automatically loaded by \fBdlopen()\fR. The directory search path used to find
\fIpathname\fR and any dependencies can be affected by setting the environment
variable \fBLD_LIBRARY_PATH\fR. Any \fBLD_LIBRARY_PATH\fR variable is analyzed
once at process startup. The search path can also be affected from a runpath
setting within the object from which the call to \fBdlopen()\fR originates.
These search rules will only be applied to path names that do not contain an
embedded \fB\&'/'\fR. Objects whose names resolve to the same absolute path
name or relative path name can be opened any number of times using
\fBdlopen()\fR. However, the object that is referenced will only be loaded once
into the address space of the current process.
.sp
.LP
When loading shared objects, the application should open a specific version of
the shared object. Do not rely on the version of the shared object pointed to
by the symbolic link.
.sp
.LP
When building objects to be loaded on a new link-map list, some precautions
need to be taken. In general, all dependencies must be included when building
an object. Also, include \fB/usr/lib/libmapmalloc.so.1\fR before
\fB/lib/libc.so.1\fR when building an object.
.sp
.LP
When an object is loaded on a new link-map list, the object is isolated from
the main running program. Certain global resources are only usable from one
link-map list. A few examples are the \fBsbrk()\fR based \fBmalloc()\fR,
\fBlibthread()\fR, and the signal vectors. Care must be taken not to use any of
these resources other than from the primary link-map list. These issues are
discussed in further detail in the \fILinker and Libraries Guide\fR.
.sp
.LP
Some symbols defined in dynamic executables or shared objects can not be
available to the runtime linker. The symbol table created by \fBld\fR for use
by the runtime linker might contain only a subset of the symbols that are
defined in the object.
.sp
.LP
As part of loading a new object, initialization code within the object is
called \fBbefore\fR the \fBdlopen()\fR returns. This initialization is user
code, and as such, can produce errors that can not be caught by \fBdlopen()\fR.
For example, an object loaded using \fBRTLD_LAZY\fR that attempts to call a
function that can not be located results in process termination. Erroneous
programming practices within the initialization code can also result in process
termination. The runtime linkers debugging facility can offer help identifying
these types of error. See the \fBLD_DEBUG\fR environment variable of
\fBld.so.1\fR(1).
.sp
.LP
Loading relocatable objects is an expensive operation that requires converting
the relocatable object into a shared object memory image. This capability may
be useful in a debugging environment, but is not recommended for production
software.
